Coal plants, landfills, and cows all release methane, a powerful greenhouse gas. But in a new study, researchers have highlighted yet another source of methane emissions: the pit latrines used by about 1.8 billion people around the world.

Pit latrines are crucial for sanitation in developing or rural areas without modern toilets. As the excrement decomposes, methane is released. But the amount of methane is unclear because emissions depend on local factors such as the groundwater level. Since the United Nations has declared improved sanitation a major goal for 2030, “it is vital for policymakers to have a comprehensive understanding of [greenhouse gas] emissions from on-site sanitation systems,” the authors write in Environmental Science & Technology.

To come up with an estimate, the researchers studied water table data and sanitation surveys for 21 countries. Based on latrine use trends from 1998 to 2011, they then tried to predict usage in 2015.

In 2000, pit latrines worldwide released about 5.2 teragrams of methane, or about 2 percent of anthropogenic methane emissions, the study authors say. But because of China’s ongoing urbanization and wastewater system improvements, the team predicts that the number will drop to 3.8 teragrams in 2015. Pit latrine use will rise in African countries such as Ethiopia as they work toward better sanitation, but not enough to offset the reduction in China. Overall, China, Bangladesh, and India will be the biggest emitters.

One way to reduce methane emissions would be to install other sanitation systems such as composting toilets. In Asia, using composting toilets instead of pit latrines would cost an extra $46 to $97 per ton of equivalent carbon dioxide emissions eliminated; in Africa, that figure would range from $57 to $944 per ton. But researchers still need to get their hands dirty and measure the methane released from pit latrines and composting toilets directly, the authors say. — Roberta Kwok | 24 July 2014